With the advent of digital multimedia data and digital multimedia data distribution, protection of such digital multimedia data against unauthorised copying and dissemination have become an issue for multimedia data publishers and authors. One technique used to identify ownership is to embed a pattern or patterns into the data, such that the embedded pattern is typically imperceptible to an observer. Such a pattern is called a watermark pattern, or simply a watermark. The presence of the watermark pattern can be detected in copied data by the owner of the original data, thereby proving their ownership.
A difficulty arises with respect to embedding watermark patterns into the data in such a way that the watermark patterns are both imperceptible to the human senses, and for each embedded watermark pattern to be reliably detected. In general, the watermarked data may be transformed or processed, making the detection of the watermark patterns more difficult to reliably detect. Processing that is often applied to image data is printing and then subsequent scanning of the printed image. During the printing and scanning processes, the high frequency components of the watermark pattern are often attenuated. Other processing that typically adversely affects the reliable detection of the watermark pattern includes interpolation during image rotation or scaling, or even malicious attack.
Several watermarking schemes use spread-spectrum signals as watermark patterns. Two advantages of using spread-spectrum signals as watermark patterns are the very sharp peaks obtained when using correlation to detect the watermark pattern, and also the presence of relatively large high-frequency components, which are generally less perceptible than low frequencies.
Because images commonly contain a predominance of low frequencies, direct correlation of a watermarked image with the spread-spectrum embedding watermark pattern will be dominated by the low frequencies of the image, resulting in a blurred correlation image in which correlation peaks, corresponding with the positions where the watermark patterns were embedded in the image, may not be discernible.
A related problem is the problem of correlation of data features with the watermark pattern during correlation, also causing interference. While usually not a problem with pseudo-random noise watermarks, this can be a problem with watermark patterns based upon logarithmic radial harmonic functions. For example, if the features of an image contain strong curved fringes, such as an image of spiral sea shells, additional noise may be caused during correlation of such an image with a logarithmic radial harmonic function based watermark pattern, which may obscure the correlation peaks.
Occasionally special forms of correlation are used to address this problem and restore the peaks.
One such special form of correlation is frequency pre-emphasis of the watermarked data before correlation with the watermark pattern. A linear or quadratic scaling factor applied to the high frequencies will reduce the influence of the data's low frequencies, and increase the influence of the flat spectrum of the watermark patterns, resulting in sharp, visible detection peaks.
Another special form of correlation often used is phase correlation in which all frequency components are set to a constant amplitude in the Fourier domain during correlation of the watermarked data with the watermark pattern. Because phase correlation weights all frequencies equally, it has the additional advantage of restoring the high frequency components of the embedded watermark patterns where they have been attenuated.
Although very effective in enhancing correlation peaks and reducing interference from the data, phase correlation and frequency emphasis are inherently applied equally over all data samples of the watermarked data, and are thus completely insensitive to any local variations in the watermarked data.